1. Field of the Invention
The present invention relates to a built-in-circuit substrate including an electrical circuit which has been built in and configured to receive an RF signal, and relates to a composite module which includes the built-in-circuit substrate.
2. Description of the Related Art
A known ceramic electronic component 100 includes an inner electrode 102 that is formed inside a ceramic multilayer body 101; and an outer electrode 103 for an external connection that is formed on a surface of the ceramic multilayer body 101, as illustrated in an example of an electronic component of FIG. 6 (Japanese Unexamined Patent Application Publication No. 2007-281400). The outer electrode 103 of such a ceramic electronic component 100 is mainly formed of an underlying metal layer 104 including Cu or the like that is formed on a surface of the ceramic multilayer body 101; a nickel (Ni) layer 105 that covers the underlying metal layer 104; and a tin (Sn) layer 106 that covers the Ni layer. In this case, the Ni layer 105 is provided as a layer having little diffusion to the underlying metal layer 104, and the Sn layer 106 is provided as a layer for improving wetability of solder when the ceramic electronic component 100 is connected, for example, to a mother substrate or the like with solder.
Inside the above-described electronic component 100, there is a built-in-circuit substrate in which a filter circuit or the like, which is formed of a capacitor element and an inductor element, is formed in the ceramic multilayer body 101, for example. In the case where such a built-in-circuit substrate is installed in an electronic appliance that uses a high-frequency signal (RF signal) such as a cellular phone, the RF signal flows to an outer electrode formed on the built-in-circuit substrate.
In such a manner, when an RF signal flows to an outer electrode, the RF signal attempts to flow to the Sn layer, which is the outermost layer of the outer electrode, due to the skin effect. However, because the Sn layer is generally formed so as to be thin, there is a case in which the RF signal flows into the Ni layer positioned below the Sn layer.
The Ni layer, which is formed of a magnetic material (ferromagnetic material), has a magnetic flux density-magnetic field strength characteristic (B-H characteristic) that is expressed by a magnetic hysteresis curve as illustrated in FIG. 7. Here, a region R1 in which the magnetic flux density is saturated with respect to magnetic field strength and a non-saturation region R2 other than that region exist. When an RF signal flows into the non-saturation region R2, particularly in the case where a plurality of RF signals having different frequencies is simultaneously input, intermodulation distortion (IMD) is generated.
Therefore, as described above, in the case where the built-in-circuit substrate is used in a communication system in which a plurality of RF signals having different frequencies is simultaneously used, there is a risk that communication characteristics are degraded by noise generated from the built-in-circuit substrate, and therefore, a technology is demanded for preventing generation of noise caused by intermodulation distortion.